Several attempts have been made during the past years to establish a
suitable criterion to assess the degree of complexity in various
scientific fields [1]}, [2]}, [3]}, [4]}. However, regardless of
all these efforts done so far, there is still no agreement on an
accurate definition. The structure of an ideal gas possesses low
information content as its atoms are arranged symmetrically
(following specified symmetry rules). However, a large amount of
information is required to characterize the framework of an
ideal gas due to the irregular arrangement of atoms. Being the
simplest models, both the above-mentioned systems (i.e., perfect
crystal and ideal gas) manifest zero complexity. Since these models
are extreme in the scale of information and arrangement, therefore
the principle of complexity must comprise some other terms. One of the
earliest attempts of describing complexity was based on entropy and
information of the system [5]}. López-Ruiz
et al. [3]} illustrated the notion of
complexity on the basis of disequilibrium, which is maximal for a
perfect crystal and vanishes for an ideal gas. This novel concept
assigned the same value of complexity to both the systems. It is
notable that the concept of complexity in accordance with
López-Ruiz's approach has been suggested already for the
self-gravitating cosmic structures
[7]}, [8]}. However,
being a significant constituent, pressure of the fluid distribution
plays a crucial part in determining the physical characteristics and
evolution of stellar structures. Thus, the definition of complexity
must also encompass the term pressure.
